A variety of kinds of gas barrier materials have heretofore been proposed. In recent years, in particular, there have been proposed gas barrier materials obtained by crosslinking a polycarboxylic acid polymer with polyvalent metal ions from the standpoint of environmental friendliness and not being affected by humidity conditions.
For example, there have been proposed a gas barrier film obtained by applying a layer that contains a metal compound onto the surface of a layer of a formed material comprising a mixture of polyalcohols and at least one poly(meth)acrylic acid polymer selected from the group consisting of a poly(meth)acrylic acid and a partly neutralized product of the poly(meth)acrylic acid (patent document 1), and a gas barrier film obtained by forming a gas barrier coating on at least one surface of a thermoplastic resin film, the coating being formed of a polyacrylic acid that is crosslinked with a crosslinking agent containing an epoxy compound having three or more epoxy groups per a molecule thereof, and the crosslinking agent being contained in an amount of 1 to 100 parts by mass per 100 parts by mass of the polyacrylic acid (patent document 2).
The gas barrier materials disclosed in the above patent documents 1 and 2 must be crosslinked to a high degree at a high temperature of not lower than 150° C. or by being heated for extended periods of time accompanied by such problems as seriously affecting the plastic base material, and requiring a dip treatment or a spray treatment at the time of ionic crosslinking with metal ions, lowering the productivity, and consuming large amounts of energy and water. Besides, the gas barrier materials are not still satisfactory in regard to flexibility and retort resistance.
As a gas barrier laminated film on which dry-printing can be effected at a relatively low temperature, further, there has been proposed a gas barrier laminated film comprising a layer (a) formed from a coating solution (A) containing a polycarboxylic acid polymer without heat treatment and a layer (b) formed from a coating solution (B) containing a water-soluble polyvalent metal salt and an aqueous resin, the layer (a) formed from the coating solution (A) and the layer (b) formed from the coating solution (B) forming at least a pair of lamination units abutting to each other (patent document 3).
The gas barrier laminated film disclosed in the above patent document 3 can be dry-printed at a relatively low temperature; i.e., the gas barrier laminated film is obtained without affecting the base material. With this laminated film, however, the polycarboxylic acid polymer that is applied first is fixed and does not sufficiently migrate into the layer (a) of the polyvalent metal layer. Therefore, the crosslinking with polyvalent metal ions is not attained to a sufficient degree, and the gas barrier property is not still satisfactory unless the ionic crosslinking ratio is enhanced by conducting a dip treatment or a spray treatment which is the conventional method as the treatment after the printing.
The present inventors have, further, proposed a gas barrier material having a barrier layer comprising a polycarboxylic acid polymer, the surface layer of the barrier layer forming a chemical bond stemming from an isocyanate group, and the content of nitrogen in the surface layer being at least 1 atom % or more per the total amount of carbon, oxygen and nitrogen (patent document 4).